TSref

The documentation of #14667 is missing.

The stdWrap property filelink (= function filelink) has a property called icon_thumbSize.

Property: icon_thumbSize
Data type: string /stdWrap

Description:
Defines the size of the preview image.
That is the value of the "&size=" parameter in the scr attribute inside the img tag, which links to "t3lib/thumbs.php"
"icon" needs to be set for the option to take effect.
File extension of the image file must be part of "icon_image_ext_list".
"iconCObject" may not be set.

One and two values are possible, see the examples:
Examples:
icon_thumbSize = 150
icon_thumbSize = 40x40
The second produces an image tag like:
EXAMPLE STILL UNTESTED!
<img src="../../../thumbs.php?&dummy=...&file=login.gif&size=40x40&md5sum=...">

It is defined in typo3/sysext/cms/tslib/class.tslib_content.php, function filelink().
Here are excerpts of the code:

if ($conf['icon']) {
  if ($conf['iconCObject']) {
    ...
  } else {
    $thumbSize = '';
    if ($conf['icon_thumbSize'] || $conf['icon_thumbSize.']) {
      $thumbSize = '&size=' . isset($conf['icon_thumbSize.'])
      ? $this->stdWrap($conf['icon_thumbSize'], $conf['icon_thumbSize.'])
      : $conf['icon_thumbSize'];
    }
    ...
   $icon = 't3lib/thumbs.php?dummy=' . $GLOBALS['EXEC_TIME'] . '&file=' .
     rawurlencode('../' . $theFile) . $thumbSize . $md5sum;

   }
   $icon = '<img src="' . htmlspecialchars($GLOBALS['TSFE']->absRefPrefix . $icon) . '"' .
     $this->getBorderAttr(' border="0"') . '' . $this->getAltParam($conf) . ' />';

   return $icon . $file . $size;
}